This invention relates to a process of manufacturing flat glass. The invention particularly relates to a process involving drawing glass upwardly as a continuous ribbon from a bath of molten glass via a drawing meniscus located at the surface of such bath in a drawing zone into which currents of glass flow in respectively opposite directions along such surface. One such surface current, the forward current, flows into such zone from a region to which molten glass is continuously supplied. The invention also relates to apparatus for use in carrying out this process.
In the drawing of flat glass, numerous problems are encountered in controlling the thermal conditions associated with the supply of molten glass and existing in the environment to which the glass is exposed while being drawn. Such control must be exerted in order to produce flat glass with acceptable optical and surface properties. Ideally, the faces of the drawn ribbon of flat glass should be truly flat and parallel at all positions across the glass surface. In practice, this ideal quality can not be realized under industrial conditions.
It is possible, by employing various known measures to reduce the disparities in temperature which tend to arise between one part of the glass and another, to produce flat glass of reasonably high quality insofar as its geometry is concerned. The environmental influences on flat glass formation are, however, numerous and subtle and in the flat glass manufacturing industry efforts are being continuously made to discover new techniques either for the purpose of still further improving the realizable quality of the flat glass or for enabling flat glass of a given quality to be produced more easily or cheaply.
The quality of the end product is particularly liable to be influenced by the thermal and rheological conditions which prevail in the region of critical formation of the ribbon, i.e., in and immediately above the drawing zone where the molten glass is drawn upwardly from the surface of the supply of molten glass and into the ribbon via a meniscus, which is established at the bottom of the glass ribbon by the drawing action on the glass.
It is well known that it is important for the position of the meniscus to be stabilized because otherwise the geometry of the drawn ribbon would be highly irregular. In order to stabilize the position of the meniscus, it is the practice in some glass drawing processes of the kind with which the present invention is concerned to provide a draw bar which is immersed in the bath of molten glass a short distance below the surface of such bath at the drawing zone. The forward surface current of molten glass flows over that draw bar and directly feeds the front side of the glass ribbon, i.e., the side facing toward that region to which molten glass is continuously supplied to the bath, whereas the glass which feeds the rear side of the ribbon initially flows under the draw bar and then flows back over it in the opposite direction.
However, it is not only the position of the meniscus which is important but also the viscosity profile of the molten glass forming the meniscus at any given moment. There is a natural tendency for thermal gradients to exist in the meniscus. For example, there is often an appreciable difference between glass flowing into the meniscus at the central part of the length of the drawing zone, measured in a direction normal to the direction of flow of the surface currents feeding the main faces of the ribbon, and the molten glass flowing into the end region of the drawing zone and thus feeding the margins of the ribbon. There is also often a tendency for the temperature of the forward surface current flowing via the meniscus into the front face of the ribbon to be higher than the molten glass flowing via the meniscus into the rear side of the ribbon.